I've analyzed all the rooms of the original PoP1 game that are not in use.
(I looked at the DOS port, but Apple II has the same rooms.)
I've marked the most interesting unused rooms with yellow.

As you can see, one of the things I found is that room 20 of level 13 still contains an old version of room 20 of level 12.
Mechner copied the entire level 12 to level 13, then he didn't use room 20 in level 13, and he later modified room 20 of level 12 to be more difficult.
So, I discovered a bit of PoP development history.

With a small red potion!
It wasn't really useful, because it's quite hard to lose only one HP instead of dying on that level.
(And the prince's HP will regenerate anyway before the shadow fight, due to a special event that may be a bug.)
In the new version of the room, it's actually easier to lose one HP, with those loose floors.

Level 8, room 11 (with the two level doors) reminds me of a scene in Goldeng's video about that level... (around 1:07)
Unfortunately, his videos are "unavailable"!